This invention relates to a signal mixing apparatus utilizing digital signal processing technology for mixing digital signals of plural channels and, more particularly, to a digital mixer capable of mixing inputs of a large number of channels with a relatively small amount of wirings.
In a prior art signal mixing apparatus utilizing digital signal processing technology used for mixing data of a relatively small number of channels, a bus is formed in such a manner that a memory is shared by plural DSPs (digital signal processors). FIG. 2 shows an example of this prior art signal mixing apparatus. In the example of FIG. 2, digital input data ch1 to ch4 of respective channels are applied to DSPs 10 and 12 for signal processing and, after holding these data in a memory 14, these data are sequentially read out and provided as mixed outputs after applying signal processing to them.
There is another prior art signal mixing apparatus using adders for mixing. FIG. 3 shows an example of this type of signal mixing apparatus. In this signal mixing apparatus, digital input signals ch1 to ch8 are applied to corresponding one of DSPs 18, 20, 22 and 24 for signal processing and thereafter are sequentially added together by adders 26, 28 and 30 and then the sum signal of all the digital input signals ch1 to ch8 is processed by a DSP 32 for outputting a mixed signal.
The prior art signal mixing apparatus of FIG. 2 is effective in a case where the number of channels or buses is relatively small. Since, however, outputs cannot be obtained from the DSP 10 and DSP 12 at the same timing, the number of times the DSPs 10 and 12 can access the memory 14 within one sampling period is limited with a result that a large number of channels or buses cannot be provided in this type of signal mixing apparatus. Further, if moduled circuits are adopted for inputs of respective channels, these moduled circuits will have to be controlled by supplying different timing signals to them because these moduled circuits will not be able to access the memory at the same timing. This will require a complicated circuit design. Besides, as a problem before considering merits and demerits of using moduled circuits, it is practically impossible for this type of digital mixer, due to inherent limitation on the speed of carrying out addition, to perform addition of data of a large number of channels for which adoption of moduled circuits is considered suitable.
The DSPs 18, 20, 22 and 24 of the signal mixing apparatus of FIG. 3 can output signals at the same timing and, accordingly, a larger number of channels or buses can be employed than in the signal mixing apparatus of FIG. 2. This signal mixing apparatus however requires a large amount of wirings since, for example, a complex wiring is required for a single DSP. Besides, in a case where moduled circuits are employed in this digital mixer, the capacity of input wirings of a master module to which the moduled circuits are to be connected is limited and fixed and, further, each one of the moduled circuits occupies a certain number of wirings and, accordingly, it is theoretically impossible to change the number of modules without limitation as in an analog mixer. It is not desirable either to increase the capacity of the master module to a large extent so that this type of signal processing device is not preferable in respect of utilizing the module.
It is, therefore, an object of the invention to provide a digital mixer capable of mixing input data of a large number of channels with a small amount of wirings and also capable of employing moduled circuits and changing the number of channels without difficulty.